This invention relates to radio frequency transmissions and, more particularly, to a microwave relay system having the ability to achieve phase-locked operation between the transmitter and receiver by using dual related pilot tones.
In cable television systems (usually referred to as CATV systems), it is often necessary or desirable to use microwave transmission links to relay the cable signal from one cable site to another where there are subscribers to the service. It is conventional in such microwave relay systems to use single sideband modulation to reduce the band width requirements and to suppress the microwave carrier to improve the power efficiency.
Synchronous demodulation with the carrier is required at the receiver to recover the original signal. When the carrier is suppressed during transmission, the demodulation carrier must be provided by a local oscillator at the receiver. In such cases, there is inevitably some slight frequency difference between the microwave carrier at the transmitter and the locally generated microwave carrier at the receiver. This will cause the demodulated video signals at the receiver to differ slightly from their corresponding frequencies at the transmitter. When this happens, and when the video signal at the transmitter has been locked to a strong locally broadcast video signal which is also available at the subscriber's receiver, interference known as "co-channel interference" will result. One way to avoid this problem is to provide electromagnetic shielding at the subscriber's receiver. However, this is expensive if the shielding is required for all of the television sets of a CATV system. To avoid this expense, it is necessary that there be no frequency difference between the transmitted and relayed video signals. This means that the local oscillator at the receiver must be phased locked to the microwave carrier at the transmitter.
U.S. Pat. No. 3,778,716 of Stokes addresses this specific problem. Stokes provides a pilot signal which is transmitted with the television signals over the microwave link. At the receiver the pilot signal is compared to the local oscillator in a phase-locked loop that synchronizes the local oscillator frequency to the pilot frequency.
In practice, this system has serious disadvantages. Since the local oscillator and the pilot tone are compared to each other in the receiver phase-lock loop, the pilot frequency must be related to the suppressed carrier. This limits the frequencies that can be used for the pilot tone. Also, the pilot tone-local oscillator comparison is not possible if the pilot tone is not received for some reason. As a result, no local carrier will be produced and no demodulated signal will be generated for display at the receiver if the pilot signal is lost.